The blade is the component of a wind turbine that is most vulnerable to being hit by lightning. Numerous factors make the design of an adequate system that prevents damage from getting hit by lightning complex: its great length, the combination of non-conductive materials (such as fiberglass) with other conductive ones (such as carbon fiber), low resistance to the heat of the resins utilized, etc . . .
The need to equipotentialize all of the conductive elements of the blade and connect them to the lightning arrester system for the rest of the wind turbine is covered in the existing bibliography on lightning strikes.
Carbon fiber, as a conductive material, must be equipotentialized with the lightning arrest system. The problem with leaving conductive materials isolated is the very elevated difference in power that is created between them due to the induction phenomena originated by the lightning bolt while it passes through the lightning arrest system. This difference in power can give way to skipping the arc, which would be a fatal situation for the fiber laminate of the blade.
Most blade designs include a lightning protection system that consists of a series of receptors placed in the area closest to the tip of the blade, that are joined by a cable that descends through the interior of the blade to a system that is located in the hub of the wind turbine, which guides the lightning bolt to its base. The system must make it such that the impact of the lightning bolts is attracted to the receptors, in order to prevent them from hitting other more exposed parts of the blade. Once the lightning is captured, the system must make it such that the lightning is guided to the base of the blade without its great energy causing damages to the structure of the blade.
Different solutions are known based on this kind of typology. They are shown in patents like DE4436197, WO0177527, U.S. Pat. No. 6,612,810, WO9607825, EP0707145 or the applicant's patents ES2255436 and ES2255454.
On the other hand, the large size reached in the latest blade designs is causing the designs to be carried out where the blade is divided into several sections that are assembled on the site of the wind turbine. In this manner, the logistics and transport of the machine assembly is simplified and sites where it is not viable to transport large blades can be accessed.
There can be two kinds of blade joint systems: chemical joints (through adhesive means) or mechanical joints (through bolting solutions). Given how sensitive adhesive joints are to environmental conditions, the joints of blades based on bolting systems are more robust and appropriate for on field assembly.
Patents EP 1 584 817 and ES 2 265 760 from the applicant show two sectional blade solutions whose mechanical joints are done with bolting solutions.
There are other known sectional blade solutions whose sections are joined mechanically through bolted unions like those shown in patents US20030138290 and DE3109566.
Therefore, the inclusion of bolted joints between blade sections is achieved through the inclusion of metal elements in the blade joint.
These metal elements directly affect the behavior of the blade when faced with lightning strikes, given that they become potential lightning strike attraction points, and therefore must be considered in the design of the lightning strike blade protection system, given that the impact of lightning bolts on the bolted joint could damage it (generate points where cracks could start, damage to threaded joints, damages to possible monitoring systems, jumps of arcs between different metal elements, etc . . . )
A basic protection solution shall consist of connecting the metal elements to the blade's grounding line, but connecting them to the ground makes them more susceptible to a lightning strike. Not connecting these metal elements to the grounding cable reduces risk, but it must be taken into account that, if a lightning strike were to occur, the current could only be passed on to the grounding cable through an electric arc, with subsequent damage to the joint.
There are also known lightning strike protection system solutions like European Patent EP 2 019 204, which proposes the placement of a receptor, like a clamp, on the outer surface of the blade's composite material (embedded in the laminate or placed afterwards on top of the composite). This solution behaves in the same manner as a typical tip receptor, with only the form of the receptor changing, having a larger surface than typical receptors. However, these solutions, because they are on the surface of the shell, do not solve the problem of lightning strikes on the connectors of the blade sections, given that the placing of receptors like the one shown on the side of the blade joint would not eliminate the possibility of strikes to the metal elements of the blade joint.